Solid ink or phase change ink printers conventionally use ink in a solid form, either as pellets or as ink sticks of colored cyan, yellow, magenta and black ink, that are inserted into feed channels through openings to the channels. Each of the openings may be constructed to accept sticks of only one particular configuration. Constructing the feed channel openings in this manner helps reduce the risk of an ink stick having a particular characteristic being inserted into the wrong channel. U.S. Pat. No. 5,734,402 for a Solid Ink Feed System, issued Mar. 31, 1998 to Rousseau et al.; and U.S. Pat. No. 5,861,903 for an Ink Feed System, issued Jan. 19, 1999 to Crawford et al. describe exemplary systems for delivering solid ink sticks into a phase change ink printer.
After the ink sticks are fed into their corresponding feed channels, they are urged by gravity or a mechanical actuator to a heater assembly of the printer. The heater assembly includes a heater that converts electrical energy into heat and a melt plate. The melt plate is typically formed from aluminum or other lightweight material in the shape of a plate or an open sided funnel. The heater is proximate to the melt plate to heat the melt plate to a temperature that melts an ink stick coming into contact with the melt plate. The melt plate may be tilted with respect to the solid ink channel so that as the solid ink impinging on the melt plate changes phase, it is directed to the reservoir for that color. The ink stored in the reservoir continues to be heated while awaiting subsequent use.
Each reservoir of colored, liquid ink may be coupled to a print head through at least one manifold pathway. The liquid ink is pumped from the reservoir to the print head as the print head demands ink for jetting onto a receiving medium or image drum. The print head elements, which are typically piezoelectric devices, receive the liquid ink and expel the ink onto an imaging surface as a controller selectively activates the elements with a driving voltage. Specifically, the liquid ink flows from the reservoirs through manifolds to be ejected from microscopic orifices by piezoelectric elements in the print head.
Ink-jet printing systems commonly utilize either direct printing or offset printing architecture. In a typical direct printing system ink is ejected from jets in the print head directly onto the final receiving medium. In an offset printing system, the print head jets the ink onto an intermediate transfer surface, such as a liquid layer on a drum. The final receiving medium is then brought into contact with the intermediate transfer surface and the ink image is transferred and fused or fixed to the medium.
In some direct and offset printing systems, the print head may move relative to the final receiving medium or the intermediate transfer surface in two dimensions as the print head jets are fired. Typically, the print head is translated along an X-axis while the final receiving medium/intermediate transfer surface is moved along a Y-axis. In this manner, the print head “scans” over the print medium and forms an image by selectively depositing ink drops at specific locations on the medium.
One object of the control strategy is to avoid the printing system, and, in particular, the print head reservoir, running out of ink while trying to print. Prior known systems typically supply a sensor in the reservoir to indicate when the ink levels therein drop below a threshold level. When the ink drops below the threshold, the ink supply control system melts more of the solid ink supply until the reservoir refills to an appropriate supply level. Detecting an ink supply deficiency, melting the solid ink in response to the deficiency, and refilling the reservoir to a supply level with the melted ink is commonly referred to as an “ink melt duty cycle.”
One problem that is faced during imaging operations is maintaining an adequate supply of ink in the reservoir. Running a print head reservoir dry can damage the print head mechanism. Even if the print head mechanism is not damaged, the print head may have to be re-primed once the reservoir is refilled or replaced. In addition, maintaining adequate amounts of liquid ink in the print head reservoir may become more difficult as throughput rates for liquid ink print heads increase.
In order to avoid exhaustion of the ink supply in the reservoir, conventional systems typically pause or stop printing when a reservoir sensor indicates that the ink level in the reservoir has reached or passed the threshold level. Printing operations are paused or stopped until the ink level in the reservoir is replenished to at least the threshold level. Thus, during high throughput printing operations, a printer may have frequent and/or intermittent delays to allow the reservoir to be continually replenished thereby causing the printing rate to fall below specifications.